Apex domains cannot have CNAME records.
Question and response
Question
dig portsdns.net. NS
Answer
portsdns.net. 5 NS a.portsdns.se.
portsdns.net. 5 NS b.portsdns.net.
Authority
The response has no authority records.
Additional
a.portsdns.se. 5 A 45.54.35.101
b.portsdns.net. 5 A 94.16.23.1
a.portsdns.se. 5 AAAA 2607:f740:e009:0:0:0:0:1
b.portsdns.net. 5 AAAA 2a00:11c0:3d:0:0:0:0:1
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0